1. Field of the Invention
This invention relates to a method for compensating polarization mode dispersion, a polarization mode dispersion compensator and an optical fiber communication system, which are preferably employed in high speed optical fiber communication system and the like.
2. Description of the Prior Art
In high speed optical communication fiber system of 10 G bits/sec and over, the polarization mode dispersion becomes conspicuous as the communication path is elongated, so that the wave distortion of an optical signal under communication also becomes large to degrade the optical signal and thus, increase the bit error rate. As a result, the optical communication can not be realized under good condition. In this point of view, in order to realize the optical communication over long communication path under good condition, it is required to compensate the polarization mode dispersion by some kind or another means. In contrast, the polarization mode dispersion is changed with time when some optical fibers are employed in order to realize the optical fiber communication, so that it is required to compensate the polarization mode dispersion with time under the optical communication.
Conventionally, the compensation for the polarization mode dispersion has been carried out as described in Koichi Takiguchi, “Polarization mode dispersion (PMD) compensation device”, Optoelectronics, December, 2002, pp66-70. According to the conventional document, a polarization mode dispersion compensator with a polarization controller therein is employed and installed in front of a receiver or in the communication line. Then, the degree of polarization or a specific frequency component of an output signal from the compensator is selected as a monitoring signal, and the polarization controller is controlled under feedback so that the output of the monitoring signal becomes maximum.
In the feedback control, however, a complicated circuit is required to extract and select the monitoring signal, resulting in the complication of the total structure of the compensator. Moreover, since it is required that two or over parameters are controlled, the algorithm becomes complicated, so that the speed of response is lowered.